To achieve the data densities required in modern telecommunications equipment, internal data interfaces are now very high speed differential serial data with implicit clock and frame alignment. One approach requires that all data signals are synchronous to a system reference clock and frame signal that is distributed to all cards within a piece of equipment separately. Each receiving element requires a Clock Data Recoverer (CDR) which uses the edges in the incoming data to recover an independent receive clock, using the local system clock as a guide. The recovered clock and data are de-serialised and the content can then be aligned to the system frame.
Not all systems, or indeed all technologies (ASIC/FPGA, for example) provide a CDR function. For cost reasons, some systems or technologies only offer a “dynamic phase aligner” (DPA) in place of the CDR. The DPA does not supply the recovered clock signal, but adapts the incoming data to the system clock using a FIFO (first in, first out) memory. Thus, DPAs are only able to cope with a certain amount of “wander”, where the bit transitions in the incoming signal are not correctly aligned with the system clock; this can be due to the inherent delays in transmission over a line, the delays through input or output buffers, noise, etc. These delays are temperature and supply dependent, so will vary over time.
The DPA provides a “wander buffer” using the area of FIFO memory in which it stores data before it is aligned with the system clock; in SDH/SONET systems it is common to have wander buffer capable of holding sufficient data to cope with a wander in either the fast or slow directions of 20 ns. However, given the cost implications of providing sufficient memory to do so, some manufacturers are only providing 2.5 ns of wander buffer in order to reduce costs.
A problem with this is that wander is cumulative; from a given device being brought into service it is generally only possible to track the maximum amount of wander so, if wander accrues in a single direction over a period of time, the wander buffer may fill and so data be lost. Generally, the only way to empty the wander buffer is to reset the equipment in question.